The inside of a matter, particularly a human body, can be visualized by measuring a distribution of radioactive rays, typically X-rays, transmitted through matter such as the human body. A ray distribution such as of X-rays is detected nowadays generally with a large-size image sensor using a solid state image pickup device and called a flat panel X-ray sensor. The merits of a solid state image pickup device reside in that signals indicative of an energy distribution on a plane can be directly and spatially sampled from a plurality of pixel elements disposed on the plane.
The demerits reside in that since each of the plurality of pixel elements to be spatially sampled is fundamentally an independent element and has different characteristics, an acquired image is required to be corrected in accordance with variations in the characteristics among the respective pixel elements.
The main variation in the characteristics of pixel elements as energy conversion elements is a variation in conversion efficiencies (gain) and offset values. The gain and offset are required to be corrected first, if a flat panel X-ray sensor is used as a solid state image pickup device.
Description will be given on prior art technologies. FIG. 4 is a schematic block diagram of the apparatus for correcting the gain and offset. In FIG. 4, reference numeral 1 represents an X-ray generating apparatus which is controlled by a controller having a high voltage generator to generate X-rays along a direction indicated by an arrow. An object, typically a human body 2, is on a bed 3. An X-ray image sensor 5 is used for converting an intensity distribution of X-rays transmitted through the object into electric signals. The X-ray image sensor 5 is made of a large-size solid image pickup device. Signals indicative of an X-ray intensity distribution on a two-dimensional plane can be spatially sampled from a plurality of pixel elements disposed in a two-dimensional matrix form.
In the following, this X-ray image sensor is called a flat panel sensor. A sampling pitch is set generally to about 100 μm to 200 μm to image the internal structure of a human body. The flat panel sensor is controlled by the controller (not shown). Pixels are sequentially scanned to convert charges in each pixel into voltage or current. This analog electric signal is supplied to an A/D converter 6, which converts the analog signal into a digital signal.
The A/D converted digital signal is temporarily stored in a memory 7. Data in the memory 7 is stored via a switch 8 into one of two memories 9 and 10. The memory 9 stores an image signal output from the flat panel sensor under no exposure of X-rays, as an offset fixed pattern image. The memory 10 stores an image signal obtained under exposure of X-rays.
Generally an X-ray amount measuring apparatus (also called a phototimer) is used to monitor the X-ray amount transmitted through a subject and control X-ray exposure. When the cumulative X-ray amount reaches a predetermined value, X-ray exposure is stopped. When the X-ray exposure stops, the controller scans the flat panel sensor, and image information of the subject is stored in the memory 10 via the memory 7 and switch 8. Immediately thereafter, the flat panel sensor is driven under no X-ray exposure during the period same as the subject imaging time to accumulate electric charges which are converted into a digital signal and stored in the memory 7 as an offset fixed pattern signal.
This offset fixed pattern signal is stored in the memory 9 via the switch 8 which is turned to its contact B. A subtractor 11 subtracts a value in the memory 9 at each address from a value in the memory 10 at a corresponding address and the subtracted values are stored in a memory 12.
A look-up table (LUT) 13 is a table of logarithmic value conversion used for division operation. A radiation-sensed image of a subject is stored in a memory 15 via LUT 13 and a switch 14 which is turned to a contact C.
A memory 16 stores an image signal subjected to a calibration operation of the apparatus. Also in this case, an image is sensed in the above-described manner and stored via the switch 14 turned to a contact D. In the calibration operation, an X-ray amount distribution itself is sensed without the subject 2 to obtain a variation in gains of pixel elements. The calibration operation is usually performed approximately once per day, for example, at the start of a day work. With this calibration operation, data (also called a gain image) corresponding to a variation in gains of pixel elements of the flat panel sensor is stored in the memory 16.
A subtractor 17 subtracts the gain image from the subject image to generate an image signal whose gain variation is corrected. This image signal is stored in a memory 18. This corrected image signal is subjected to succeeding image diagnosing, filing, transmission, display or the like.
A general radiographic apparatus converts the corrected image signal into a diagnosis image signal by using a gradation process, a dynamic range change process, a spatial frequency process and the like. This diagnosis image signal is supplied to an external apparatus, typically a filing apparatus and a hard copy apparatus.
An X-ray image is characterized in its very wide dynamic range. For example, a medical radiographic image has a low level signal area where X-rays were almost shielded, for example, due to metal embedded in a human body.
Such a low level signal area may be neglected because it is quite insignificant as image information. However, if the gain correction is performed for this low signal level area, the gain variation pattern (gain image) of pixel elements is superposed upon a subject image, although this area corresponds to an area having almost no incidence energy, i.e., an area without a signal level variation. Noises are therefore generated by this gain correction.
This phenomenon will be described with reference FIGS. 5A, 5B, 5C, 5D, 5E and 5F. FIGS. 5A, 5B and 5C show an image of general X-ray distributions in one-dimensional representation. For the purposes of simplicity, random noises generated during image sensing are omitted. A line 101 represents a gain variation acquired in the calibration operation. A line 102 represents an offset acquired under no X-ray exposure. A line 103 represents an image of an actual subject. It is assumed herein that the subject has a uniform radiation transmission factor distribution. FIG. 5B shows an image 104 after the offset 102 is removed from the image signal 103. FIG. 5C shows an image 105 after gain correction in which the offset-corrected image 104 is divided by the gain variation (gain image) 101. This image 105 has uniform pixel values.
FIGS. 5D, 5E and 5F show an example of image data of a radiation-sensed image having very low levels with almost no sensor sensitivity. A line 106 represents a radiation-sensed image. After the offset 102 is removed from the radiation-sensed image 106, a stable image 107 having almost constant signal values is acquired as shown in FIG. 5E. After the gain correction is performed, an image is rather obtained with noise having a pattern 108 indicative of gain variation or X-ray shading as shown in FIG. 5F.
This noise level is very low, while emphasized in FIG. 5F. Although this noise level is very low, the output image has a large variation since the logarithmic LUT shown in FIG. 4 is used. A general correction method including an offset correction and a gain correction is disclosed, for example, in Japanese Patent Application Laid-Open No. 7-72256. A problem of noise generation caused by the gain correction is not reported.